A silicon-on-insulator (SOI) wafer may include a base layer of silicon, an insulating layer comprised of silicon dioxide overlying the base layer, and an upper silicon layer overlying the silicon dioxide layer. The silicon dioxide insulating layer is often referred to as the “buried oxide” layer. Integrated circuits including a collection of transistors and other circuit elements can be fabricated in the upper silicon layer. SOI wafers offer the potential for fabricating large-scale integrated circuits (ICs) that, for example, provide high-speed operation and exhibit low power consumption.
Methods for manufacturing SOI wafers include wafer bonding and separation by implanted oxygen (SIMOX). To form an SOI wafer by wafer bonding, a silicon dioxide layer is formed on one surface of a first silicon wafer, and a second silicon wafer is then bonded to this surface (e.g., the surface over which the oxide layer has been formed). The second wafer, which may be thinned, forms an upper silicon layer that overlies a buried oxide layer. To form an SOI wafer by SIMOX, oxygen ions are implanted into a silicon wafer, and the wafer is annealed to form a buried layer of silicon dioxide within the silicon wafer. An example of a SIMOX process can be found in Matsumura et al., Technological Innovation in Low-Dose SIMOX Wafers Fabricated by an Internal Thermal Oxidation (ITOX) Process, MICROELECTRONIC ENGINEERING, vol. 66, pgs. 400-414 (2003).
One problem with SOI wafers is that the buried oxide layer may provide poor etch resistance (during, for example, the formation of isolation trenches). It has been suggested that silicon nitride be used as the insulating layer in an SOI wafer rather than silicon dioxide, as silicon nitride may in some instances provide better etch resistance than silicon dioxide. An example of a technique for creating an SOI wafer having a silicon nitride insulating layer is described in Meekison et al., A Transmission Electron Microscope Investigation of the Dose Dependence of the Microstructure of Silicon-on-Insulator Structures Formed by Nitrogen Implantation of Silicon, JOURNAL OF APPLIED PHYSICS, vol. 69, no. 6 (1991). Silicon nitride is, however, a poor insulator in comparison to silicon dioxide. The band gap of silicon nitride is approximately 40 percent less than that of silicon dioxide, so the electrical isolation afforded by silicon nitride is significantly less than that provided by silicon dioxide.